Rick...
When you "Navigate to" a waypoint from
OpenCPN, he does the following:
1. Create a temporary
route from
current position to selected point. The route has only two points, start and end.
2. Activate that route.
3. If a suitable output connection is available, he sends the following
NMEA messages periodically to that port.
- ECRMB
- ECRMC
- ECAPB
- ECXTE.
4. The Cross-track error (XTE) output by OCPN is calculated as for Rhumb Line Sailing, i.e. straight line on Mercator chart.
If an
autopilot is
steering the
boat, it may choose to follow GC route if it wants to. The
current GPS position and the target waypoint position are available in the messages sent above.
If the A/P uses only XTE, then it is sailing Rhumb Line.
Alternatively, if it ignores XTE, and computes a running GC course based on the target waypoint location and current ownship position, then it is doing GC Sailing.
If the A/P is doing GC Sailing, then you would see the XTE reported by OpenCPN diverging from zero, as expected.
Of course, other chart plotters may do this differently from OpenCPN.
Personally, I wonder why we care. Rare indeed would be the case in which a two-point route long enough to benefit from GC Sailing would be traveled without need to change course for other tactical reasons (weather, traffic, etc.). Could happen with a large
motor vessel, I suppose...
Hope this helps
Dave